1. Field of the Invention
This invention relates to high strength low carbon steels having good ultraworkability or a high degree of workability. Also, the invention relates to articles of such steels as mentioned above and a method for manufacturing the steels.
2. Description of the Prior Art
In recent years, there have been developed highly ductile steels for use as high strength thin steel sheets for press forming which consist of ferrite and a low temperature transformation product phase and which have a low yield ratio. However, it is known that although these steels have good stretch formability or bulging ability, they become very poor when subjected, for example, to a high degree of working such as wire drawing in which a reduction ratio is as high as about 90%. On the other hand, it is also known that eutectoid steels of the pearlite structure obtained by the patenting treatment are considerably poor in forgeability and press formability.
We have made intensive studies to obtain steels which have not only good press formability, but also excellent ultraworkability or a high degree of workability such as cold or hot wire drawing, drawing, forging and rolling. As a result, it was found that a high degree of workability could be imparted to low carbon steels as follows. The structure of low carbon steels is first converted to bainite martensite or a fine mixed structure thereof with or without retained austenite. The reversely transformed bulky austenite is transformed under given cooling conditions to give a final structure so that a fine low temperature transformation product phase consisting of acicular or elongated bainite, martensite or a mixed structure thereof with or without containing retained austenite is uniformly dispersed in the ferrite phase, thereby forming a composite structure.